This invention relates to control systems for three phase motors and, more particularly, to power leg loss protection for three phase motors.
Air conditioning equipment, including compressors and fan motors, is designed to operate utilizing a three phase power supply and a controller, usually a thermostat, that will turn the equipment on and off as needed to maintain desired temperature. These types of motors are commonly known as three phase motors and are turned on and off by a control circuit powered by a stepdown transformer. In general, such control circuits contain contact points, or movable conductors, that close to complete a circuit when they receive a predetermined threshold voltage. Three phase motors are supplied by three main power xe2x80x9clegsxe2x80x9d (L1, L2 and L3) from a three phase power supply. Two of the main power legs also supply a control power, which is typically 24 volts, to power the control circuit. Since the control circuit operates at 24 volts, only two main power legs are needed. The stepdown transformer that generates the control power is powered by L1 and L2 and converts the higher three phase transmission voltages, usually 208, 240 or 480 volts depending on the power source, into the lower voltages necessary for powering the control circuit.
Anytime there is three phase equipment there is a risk of xe2x80x9csingle phasingxe2x80x9d. Single phasing is when one of the three legs of main power is lost due to a power company problem, electrical storm or other surge capable of overloading a main high voltage transformer that feeds a particular area. When this happens, there is little that can be done to prevent the possible burn out of equipment connected to the affected power grid. In particular, because control circuits and the stepdown transformers that power them only require two power legs to operate, power can still get to the equipment even when a particular power leg is lost especially when that particular power leg is not used to power the control circuit. This situation may result in equipment burn out. Accordingly, various systems have been developed to protect equipment in case power is terminated.
One such protection system is disclosed in U.S. Pat. No. 5,761,018 to Blakely. Blakely discloses a motor control circuit that contains logic programming for a time delayed power shut off mechanism which shuts off the electrical current flowing between a three phase power source and a motor when a fault condition is detected by circuit logic. The circuit logic prevents potentially damaging input power supplied by the three phase power source from reaching the motor by monitoring the average phase current value over time. When the fault condition is detected by the circuit logic, the motor control circuit generates a trip signal to actuate a switch which shuts off the current flow from the three phase power source, thereby preventing the three phase power from reaching the motor. Blakely discloses a time delayed power shut off mechanism because the complex circuit logic cannot shut off power immediately in the event of a leg loss. In order for the system to operate, the circuit logic has to go through the steps of monitoring the three phase power flowing into the motor for a period of time, analyzing the power flow in order to detect a fault condition and finally, if a fault condition is detected, affirmatively responding by opening the switch. Therefore, the logic circuit""s detection and response cycle will result in significant time delays between the occurrence of the fault condition, the detection of the fault condition by the logic circuit and the logic circuit""s response of opening the switch as a result of the fault condition to shut off the three phase power.
While the Blakely motor control circuit performs as intended, it does not monitor for the presence of all three phases of main power. Blakely requires complex circuit logic that includes a significant amount of additional components. Furthermore, the shut off time delay that is inherent in the design may be detrimental to certain applications that involve equipment that requires a higher level of power surge protection.
Blakely also discloses an overload relay circuit connected to the three legs of main power for monitoring the current flowing between the three phase power source and the motor to detect the presence of a current overload condition. When a predetermined overload condition is present, the circuit logic directs the overload relay circuit to actuate a switch that shuts down the power supply to the motor. After certain preconditions, the switch will reset to allow the current to resume flowing for a motor restart.
The overload relay circuit is not designed for shutting down power to the motor when one of the three main power legs is lost. Of particular note, the overload relay circuit is not designed to shut down power when a main power leg not used to power the stepdown transformer is lost. Therefore, the overload relay circuit is not designed for preventing a third of potential main power leg loss conditions, including surge conditions, which can have particularly damaging consequences. Similar to the operation of the motor control circuit, the overload relay circuit design incorporates a significant time delay between the occurrence of a fault condition and the logic circuit response due to complex circuit logic. The complex circuit logic and the necessity of additional circuit components introduce packaging constraints which may make the system in Blakely unsuitable for existing three phase systems and other applications that have physical space limitations.
Also, U.S. Pat. No. 3,581,179 to Jones discloses a multiphase motor control system with overcurrent and undercurrent protection. A three-phase power source supplies power to a multiphase motor via a stepdown transformer and is monitored by a control circuit for overcurrent and undercurrent conditions. When the input currents are either above a preset low level or below a preset high level, the circuit logic passes first and second control signals to the control circuit. If either the first or second control signal is not present, the circuit logic will activate a switch to shut off power to the motor.
The undercurrent or overcurrent protection design in Jones is not directly related to the loss of power from a main power leg and does not disclose a system for shutting down power to the multiphase motor when any one of the three main power legs drops out. For example, the system is not designed to detect when main power leg L3, the leg not utilized to power the stepdown transformer and control circuit, is absent. In addition, Jones, like Blakely, discloses a complex logic circuit that goes through the steps of monitoring the input phase current, analyzing the recorded data and responding to a fault condition by actuating a switch. The disadvantage of this logic cycle is that it results in significant time delays which may make the system inappropriate for applications that involve more power sensitive equipment. As in Blakely, the complex circuit logic and the necessity of additional circuit components in the Jones system create packaging constraints which may make the system in unsuitable for applications that have physical space limitations. The system may also be unsuitable for existing three phase power supply systems.
Therefore, there is a need in the art for a system that prevents power from a three phase power source from reaching a three phase motor when any of the three main power legs is lost, particularly the leg not used to power the control circuit. There is a need in the art for a control circuit that requires three main power legs to operate so it can detect all possible main power leg fault conditions, rather than fault conditions for just two main power legs. There is a need in the art for a three phase power shut off system that does not include complex circuitry nor circuit logic that inherently creates a time delay between the occurrence of a fault condition and the power shut off response. There is also a need in the art for such a system to be operable to protect sensitive three phase loads such as building air conditioning systems. In order to monitor the presence of phase current, there is a need in the art for a system dedicated to monitoring specifically for main power leg loss conditions. Finally, there is a need in the art for this system to be economically packaged and designed to be incorporated into existing three phase power supply systems and other applications that have physical space limitations without having to modify or add significant new components on to the existing stepdown transformer or any of the other existing components.
The present invention overcomes the above-described problems in the prior art by providing an improvement for a three phase motor control circuit. The three phase motor control circuit includes a power coil and generally open relay contact points for preventing power from a three phase power source from reaching a load when a main power leg is not present. The three phase power source has first, second and third main power legs that power the load directly when not interrupted by a set of control circuit contact points. The operation of the control circuit contact points depends on control power flowing though the relay contact points. The control power is provided by a step down transformer which receives power from the first and second main power leg. A relay circuit includes relay contact points and a power coil which can close these relay contact points only when it receives enough power to reach a threshold voltage. The relay circuit is powered by the second and third main power legs and, therefore, is operable only when the third main power leg is present.
Accordingly, in operation, if the first main power leg loses power, the stepdown transformer coil will not provide power to the control circuit. If the second main power leg loses power, the stepdown transformer coil will not provide power to the control circuit and the relay power coil will not supply power to the power coil, thereby preventing power from getting to the control circuit. If the third main power leg loses power, the relay power coil will not supply power to the power coil thereby preventing power from getting to the control circuit. Hence, in all main power leg loss circumstances, particularly the loss of the third main power leg, the load is protected from single phasing conditions.